1. Field of the Invention
The embodiments of the invention generally relate to microelectromechanical systems (MEMS), and, more particularly, to systems and methods for minimizing the effects of 1/f noise in electromechanical systems.
2. Description of the Related Art
Low frequency or 1/f noise is found in such diverse places as electronic devices, emissions of quasars, highway traffic, the global temperature, and the flow of some rivers. Thus, it is not surprising that 1/f noise is also found in magnetic materials and that it is a significant problem in magnetic sensors. The 1/f noise in magnetic sensors can be either electronic or magnetic. In addition, magnetic sensors also can have Johnson, shot, and magnetic white noise. Moreover, white-noise magnetization fluctuations in magnetoresistive heads tend to be a fundamental limit on their signal-to-noise ratio.
There has been a focus on magnetoresistive sensor technology because it shows a lot of promise for producing low cost magnetic sensors. Magnetic sensors are generally passive sensors with desirable attributes for several types of applications that include insensitivity to weather conditions, the requirement of only a small bandwidth, and the unique ability to “see through” walls and foliage without attenuation. Furthermore, in military applications it is generally difficult to make a weapon or vehicle that does not include ferrous material that can be detected by magnetic sensors. Though the permanent magnetic moment of the ferrous material can be minimized by “deperming”, which is a process of reduction of permanent magnetism, the distortion of the earth's magnetic field due to the magnetic permeability is typically difficult to hide. Data from magnetic sensors can be fused with the data from other sensor modalities such as acoustic and seismic sensors to characterize or identify and track targets. Specifically, in military applications magnetic sensors can be used for perimeter defense, at check points, as part of a suite of sensors in unattended ground sensor networks, and on unmanned ground vehicles (UGVs) and unmanned air vehicles (UAVs). Moreover, magnetic sensors can also be employed to monitor rooms and passageways that have been cleared by military personnel.
The magnetic signals from military targets come from the internal motion of ferromagnetic parts and the motion of targets relative to the magnetic sensor. The latter can arise either from the motion of the target or the sensor. Both of these magnetic signals occur at low frequencies, typically less than 100 Hz. Because the earth's field is usually larger than the field generated by the target, it is generally difficult to detect the magnetic signals that occur at low frequencies, typically less than 100 Hz. Because the earth's magnetic field is usually larger than the field generated by a target, it is generally difficult to detect the magnetic target without having the field change by relative motion between the target and the sensor. At low frequencies the electric and magnetic fields are decoupled. The magnetic strength from a target at a distance greater than the target size is usually like that of a magnet dipole. Because of the relatively short detection range of magnetic sensors, a large number of magnetic sensors may have to be used if one wants to guarantee detection over a large area.
Magnetoresistance sensors are candidate low cost sensors because they can be mass produced by batch processing on silicon wafers and the drive and read out electronics are relatively simple. The resistance of a magnetoresistance sensor is sensitive to the magnitude and direction of the magnetic field. The types of magnetoresistive sensors include anisotropic magnetoresistance (AMR) sensors, giant magnetoresistance (GMR) sensors, spin dependent tunneling (SDT) sensors, magnetic tunneling junction (MTJ) sensors, and extraordinary magnetoresistance (EMR) sensors. Magnetoresistance values as large as 220% may be observed in CoFe(100)/MgO(100)/CoFe(100) MTJ sensors at room temperature. These large values may be the result of the properties of the wave function in both electrodes and in the MgO barrier.
To detect the relative motion between the target and the magnetic sensor generally requires high sensitivity in the frequency range f<1 Hz. Unfortunately, most magnetoresistance sensors tend to suffer from 1/f noise. Furthermore, there is generally a tendency for the sensors that have a larger response to magnetic fields to also have more 1/f noise. Thus, 1/f noise is a significant problem in applying magnetic sensors to military applications.
Another problem in using magnetoresistive sensors at low frequencies and at low fields is that the induced percentage in the resistance is generally small. Thus, with a single device one must accurately measure a small change in a large signal. Because of this problem, most magnetoresistive sensors have several sensors that are arranged in bridge circuits to eliminate the DC bias offset.
Anisotropic magnetoresistance sensors are probably the most sensitive, commercial, magnetoresistance sensors to use at frequencies of 1 Hz or less. This is true despite the fact that their magnetoresistance, approximately 5%, is relatively small. The reason for this is that AMR sensors have less 1/f noise than other sensors. Accordingly, there remains a need for a device that can eliminate the problem of 1/f noise in small magnetic sensors.